Thermal Fatigue Mechanism in the GG25 Gray Iron

In this paper a mechanism for fracture of GG25 Gray Iron has been presented under the non-constrained thermal shock. In order to constrain the residual stress on the Cast Iron matrix a notched sample with dimensions 5*5*20 mm from the GG25 were designed and machined. The notch had dimensions of 0.3 mm thickness and 2.5 mm depth. Then the samples were thermal shocked between cycles 900˚C and 0˚C in the ice-water and the non-oxidation slurry. Results of SEM metallographic and micro hardness show that thermal shocks can cause the Fe3C phase decomposition to carbon and ferrite. The precipitating of carbon on initial free graphite flakes can cause the growth and toughness of the graphite flakes. Then, due to difference of thermal expansion between the graphite and the ferrite, at the interface, micro cracks propagate into the ferrite phase and leads to fracture. However, dominant phenomenon for the quenched samples into the ice-water was the hot oxidation